1. Field of the Invention
This invention relates to systems for transmitting information in an alternating current electricity supply system.
2. Description of the Prior Art
In known such systems information is transmitted by distorting the supply voltage waveform. In one known system the distortion takes the form of a reduction of the peak value of the supply voltage in each of a selection of cycles of the supply voltage waveform, the selection of cycles determining the information transmitted. Unfortunately, such reduction of the peak voltage can cause appreciable disturbance to certain loads, e.g. lighting loads. Furthermore, it is sometimes difficult to distinguish between peak voltage reduction due to load switching and peak voltage reduction effected for transmitting information.
The inventors have appreciated that this difficulty could be alleviated by arranging for the distortion to take the form of a reduction of the instantaneous value of the supply voltage during the same small fractional part of each of a selection of cycles of the supply voltage waveform, said part including a time of voltage zero. However, the problem of how to effect such alteration only in selected cycles is a considerable one. This problem is high-lighted by the disclosure of U.S. Pat. No. 3,488,517 filed by James Macmillan Cowan, Frederick Edwin Brooker, Brian Nield, William James Stuart Rogers, Edmund Smith, Frank Tarpey and John Durnford on June 26, 1967. In that specification a system is described wherein voltage reductions in respective periods each including a time of voltage zero are produced to transmit information. However, the method employed to effect such reductions is to load a supply transformer in the supply system in such a manner that a direct current component is drawn from the transformer and a uni-directional flux component gradually builds up in at least one limb of the core of the transformer. As a result, after a significant period, typically fifteen seconds, the transformer core magnetically saturates for a short period during each subsequent cycle with a resultant reduction in the transformer secondary voltage in each cycle for a short period around a voltage zero. It will be appreciated that with this system, since several seconds elapses between the operation of a control circuit to introduce a direct current component and eventual magnetic saturation, it is virtually impossible to effect voltage reductions in single designated cycles. Hence determination of the information to be transmitted by selection of the cycles in which voltage reduction is effected, e.g. using a 5 from 32 code, is virtually impossible.